Plutonium-238 (Pu-238) has been used as a convenient, compact, long-lasting energy source referred to as a “radioisotope thermoelectric generator” (RTG). Specifically, Pu-238 is a plutonium isotope that has a half-life of approximately 88 years. Pu-238 spontaneously decays to form alpha particles and uranium-234, also referred to as U-234, which then decays further along the radium series to lead-206. Pu-238's decay also generates heat that may be converted into electrical power at a rate of 0.5 W per gram of Pu-238. Because of Pu-238 has a half life that is sufficiently short to generate a useful amount of power, and is sufficiently long to provide that power to a device for a useful amount of time, Pu-238 is a particularly desirable energy source for spacecraft and satellites, among other types of devices.
However, Pu-238 is not a naturally occurring isotope of plutonium, and therefore must be synthesized. The United States performed a great deal of research between the 1940's and the 1960's into chemical and/or physical methods of separating fission reactor products, e.g., plutonium, uranium, neptunium, and fission products, from one another. A relatively small number of methods employed volatility, see, for example, U.S. Pat. No. 2,785,047 to Brown et al., U.S. Pat. No. 2,833,617 to Seaborg et al., U.S. Pat. No. 2,865,704 to Jaffey et al., U.S. Pat. No. 2,882,125 to Spedding et al., and U.S. Pat. No. 3,294,493 to Jonke et al. One method, U.S. Pat. No. 2,869,982 to Brown et al., employed fractional distillation to separate plutonium from uranium. However, such chemical and/or distillation-based methods may not satisfactorily separate particular isotopes of a given element from one another. For example, if the fission reactor products contain both Pu-238 and its heavier isotope Pu-239, the two isotopes may not readily be separable from one another using chemistry and/or distillation. Pure Pu-238 may only reasonably be obtained by a method in which Pu-239 and Pu-238 are not admixed with one another.
Interest in preparing Pu-238 recently has revived because of the isotope's suitability for use in RTG's for use in spacecraft. For example, U.S. Pat. No. 6,896,716 to Jones discloses that schemes are known for producing Pu-238 by irradiating a neptunium-237 (Np-237) target with thermal neutrons in a nuclear reactor. Specifically, Jones discloses that such irradiation of an Np-237 target produces Np-238, which decays via β-decay into Pu-238 with a half life of 2.12 days. Jones points out that such a scheme may result in generation of both higher and lower isotopes of plutonium. For example, Jones discloses that the Pu-238 produced in the target itself becomes a target for producing higher plutonium isotopes, such as Pu-239 and Pu-240. Jones also discloses that fast neutrons may cause Np-237 to decay to form uranium-236 (U-236) and Pu-236, the latter decaying to U-232, which has a hazardous gamma-ray emitting daughter product. Jones is directed toward an alternative method of producing Pu-238 that is based on irradiating targets of a stable oxide of americium-241 (Am-241), with a high thermal neutron flux within a reactor. Jones discloses that after 20-30 days, the Am-241 is converted to curium-242 (Cm-242), which then promptly must be chemically separated, preferably within 10-20 days. The Cm-242, which has a half-life of 163 days, decays to Pu-238.
Although Jones states that the Am-241 based method is capable of preparing Pu-238 with a purity of about 95%, it is clear that the process is relatively time-consuming and includes several cumbersome steps. For example, the targets must be physically inserted into and removed from a nuclear reactor on a fixed schedule, must promptly be chemically treated, and the resulting reaction products must be allowed to decay over a period of several months to obtain Pu-238. The shortcomings of the previously known Np-237 based method are also clear, including similar requirements for inserting and removing the targets from a nuclear reactor, and likely production of one or several other plutonium isotopes besides Pu-238. Separating such isotopes from one another may be difficult. In this regard, it should be noted that each plutonium isotope produced other than Pu-238 reduces the overall yield of Pu-238, thus reducing the power that the material may generate.
Notwithstanding the methods that Jones describes, it is not believed that any production facilities for Pu-238 presently exist anywhere in the world. Although some Pu-238 has been previously produced and stored, primarily using the previously known Np-237 based method, the present supply of Pu-238 is believed to be fixed and is being consumed at such a rate that Pu-238 may not be available for RTGs to power future spacecraft and satellites.
Thus, what is needed is a practicable, efficient, low-cost, and rapid method of preparing Pu-238 with high isotopic purity.